1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting control information in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4-th generation) mobile communication standard following the IMT-2000. In late 2006, it was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Control information includes a variety of information to control communication between a base station (BS) and a mobile station (MS). The control information includes a channel quality indicator (CQI) indicating a channel condition, an positive-acknowledgement (ACK)/negative-acknowledgement (NACK) for hybrid automatic repeat request (HARQ), and/or multiple input multiple output (MIMO) related information. The MIMO related information includes a precoding matrix indicator (PMI), a rank indicator (RI), etc., for an MIMO operation. Since the control information is in regard to decoding and/or scheduling of information bits, transmission reliability is important. This is because erroneous control information results in data communication errors. Therefore, in most wireless communication systems, a control channel for control information is designed independently from a data channel.
Uplink control information is transmitted by the MS to the BS. An uplink control channel carrying the uplink control information needs to be designed by considering power consumption and mobility of the MS.
The control information can be transmitted by using two methods. In a first method, the control information is transmitted in a codeword format by performing channel encoding on the control information. The first method requires a reference signal used in channel estimation for codeword decoding. In a second method, the control information is transmitted by mapping the control information to a sequence. The second method does not require the reference signal since the control information is detected using a correlation property. Detection using the reference signal is referred to as coherent detection. Detection not using the reference signal is referred to as non-coherent detection.
A sequence may be repetitively transmitted as one scheme used for robust and effective sequence transmission. The use of a repetitive sequence decreases a code-rate, thereby decreasing a transmission rate. However, by combining the repetitive sequence, detection performance of the control information can be improved.
When the sequence is transmitted by repeating the sequence in the same format, simple implementation is possible. However, when a resource used in sequence transmission is allocated mostly in any one of a time domain or a frequency domain, it may have an adverse effect on detection performance according to mobility of the MS. In addition, when using non-coherent detection, high mobility of the MS has an adverse effect on sequence detection performance, which may result in deterioration of transmission reliability of control information.